Auto merge of #43746 - eddyb:sound-thread-local, r=alexcrichton

Check #[thread_local] statics correctly in the compiler.

Fixes #43733 by introducing `#[allow_internal_unsafe]` analogous to `#[allow_internal_unstable]`, for letting a macro expand to `unsafe` blocks and functions even in `#![forbid(unsafe_code)]` crates.

Fixes #17954 by not letting references to `#[thread_local]` statics escape the function they're taken in - we can't just use a magical lifetime because Rust has *lifetime parametrism*, so if we added the often-proposed `'thread` lifetime, we'd have no way to check it in generic code.
To avoid potential edge cases in the compiler, the lifetime is actually that of a temporary at the same position, i.e. `&TLS_STATIC` has the same lifetime `&non_const_fn()` would.

Referring to `#[thread_local]` `static`s at compile-time is banned now (as per PR discussion).

Additionally, to remove `unsafe impl Sync` from `std::thread::local::fast::Key`, `#[thread_local]` statics are now not required to implement `Sync`, as they are not shared between threads.

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